The present invention relates to a metal heating furnace installed upstream of a hot rolling mill so as to heat intermediate shapes such as slabs, billets or blooms to suitable rolling temperatures.
In the prior art metal heating furnaces, fuels are burned directly through burners in the furnaces so that intermediate shapes are heated to their suitable rolling temperatures mainly by energies radiated from luminous frames, gases and furnace walls.
In such a heating furnace, the mixture of fuel gases and combustion air from the burners is burned in a furnace space. Therefore, in a recently constructed heating furnace which has become very large in size (for instance, 10-15 meters in width and 30-50 meters in length), it becomes extremely difficult to attain uniform temperature distributions in the furnace especially because of the reasons to be described below:
(1) In the case of the side-fired furnace in which burners are extended through the side walls of the furnace, the flame temperature distribution must be uniform, for instance, over six meters from each side wall if the furnace is 12 meters in width. In the prior art direct-fired type heating furnace, the side burners are short in length and are opened at the side walls. As a result, even if the combustion mixture is forced to issue through the burners under high energies or pressures, the flames can propagate only from three to four meters at the longest. Therefore, the temperatures at the center portion of the furnace are considerably lower than those in the vicinity of side walls and consequently the temperature difference reaches as high as 100.degree. C. or higher.
(2) Fuels and combustion air issued from the direct-fired burners are mixed, ignited and burned in the free space in the furnace so that the mixing conditions vary over a wide range depending upon desired combustion capacity or requirements. As a result, there exists a wide difference in temperature between the flames themselves.
(3) In addition, the smaller the combustion capacity, the shorter the flame length becomes and the lesser the energy with which the mixture of fuel and air is issued through the nozzles becomes. As a consequence, the flames are bent and staggered; that is, the flames are distrubed by the flows and floating forces of the gases in the furnace so that the flame fronts cannot reach the center portion of the furnace. Thus, a wide temperature difference results in the furnace.
(4) In a newly constructed heating furnace which increasingly becomes large in size, a walking beam or pusher must be incorporated. In this case, many water-cooled structures must be installed in the lower portion of the furnace so as to support the intermediate shapes being heated and walking beams as well. As a consequence, the arrangement of burners is structurally limited. In addition, flames fluctuate and vary their length so that the temperature difference in the furnace is further enhanced.
(5) There has been devised and demonstrated the so-called axial burner system in which the direct-fired burners are disposed longitudinally or axially of the furnace so that the above-described problems may be solved. In this case, however, in order to avoid interference with water-cooled supporting structures in the lower portion of the furnace, the burners cannot be arranged to attain an optimum condition or environment for heating the intermediate shapes. In addition, the number of burners is limited so that the capacity of each burner must be increased. Therefore, the widthwise temperature difference results in the heating furnace and is pronounced easpecially between the portions where the burners are installed and the portions where no burner is disposed. Such temperature difference cannot be neglected especially in the case a heating furnace which is installed to heat the intermediate shape to relatively low temperature, whereby it may be rolled at relatively low temperatures.
(6) When the axial burner system is employed and if the fuel consumption is reduced from its full capacity, the flames are shortened in the longitudinal or axial direction of the furnace as with the case of the side-fired furnace. As a result, a wide temperature difference occurs; that is, no uniform temperature distribution cannot be attained.
(7) In the case of the axial burner system, in order to install burners, valleys and ridges must be formed in the furnace walls. However, such arrangement inevitably interfers with a walking beam system which is disposed in the lower portion of the furnace. As a consequence, the overall design and construction of the heating furnace becomes very complex. In addition, availability of space for other devices and equipment in the vicinity of the furnace floor is limited. Moreover, maintainability and safety factors are adversely affected.
(8) The above-described phenomena are more pronounced in a heating furnace for heating the intermediate shape to relatively low soaking temperatures between 900.degree. and 1050.degree. C. as compared with the prior art furnaces for heating them to relatively high temperatures between 1200.degree. and 1300.degree. C. so that localized hot spots or localized heat concentrations tend to occur very often. This tendency may be easily verified from the fact that as shown in the following formula radiation heat transfer is proportional to the difference between the 4-th power of the heat source surface absolute temperature and the 4-th power of the intermediate shape surface absolute temperature: ##EQU1## where Q=heat transfer rate (kcal/m.sup.2 .multidot.h),
K.multidot.F=constant, PA1 Tr=absolute temperature of surface of heat source, and PA1 Ts=absolute temperature of surface of intermediate shape.
In view of the above, the primary object of the present invention is to overcome the above and other problems encountered in the prior art metal heating furnace; that is, to attain uniform temperature distributions in the furnace and savings in energy, to reduce operating costs and to improve quality of steel stocks.
The present invention will become more apparent from the following description of preferred embodiments thereof taken in conjunction with the accompanying drawings.